Interpretive Summary: In the context of T. spiralis infection in the U.S., pigs act as the reservoir population, while sylvatic carnivores represent the target population which cannot sustain the infection in the absence of infected swine. Today, T. spiralis is virtually absent from the U.S. commercial pig population, and recent studies have shown that the risk for zoonotic transmission of sylvatic Trichinella genotypes (i.e. T. nativa, T. pseudospiralis, T. murrelli, Trichinella T-6) to pigs is negligible. Consequently, our data support the hypothesis that in the absence of a significant source of T. spiralis-infected swine, the risk of infection to wildlife hosts and the development of an independent sylvatic transmission cycle of T. spiralis is minimal.

Technical Abstract:
Pigs infected with the zoonotic parasite Trichinella spiralis were detected on a farm in Maryland during an animal welfare investigation. Sera and/or tissues were collected from 49 pigs and 3 pig carcasses (7 weeks of age to adult, mixed sex). The tissues were tested for the presence of T. spiralis muscle larvae by digestion, and the sera were tested for the presence of anti-Trichinella antibodies by ELISA. Seventeen of 50 (34%) pigs were infected with T. spiralis based on tissue digestion. Of these 17 pigs, sera were collected from 16 and analysed by ELISA. Nine of these were serologically positive, 3 had elevated OD results, and 4 were negative (indicating that they had been infected for fewer than 4 weeks). All pigs lacking muscle larvae were also ELISA negative. The farm was subsequently depopulated of pigs. Six months later, initial testing of small scavenging mammals in the environment around the farm demonstrated that T. spiralis was present in the surrounding sylvatic animal population; 41% of trapped animals were T. spiralis positive. Subsequent testing revealed that 1 year after depopulation, only 10% of trapped animals were T. spiralis positive, and after18 months, T. spiralis could not be detected in the scavenging mammal population surrounding the farm. Results of the study suggest that T. spiralis, typically transmitted only in the peridomestic rat-pig-human cycle in the U.S., was not maintained in sylvatic carnivores in the absence of infected pigs.